More precisely, the invention relates to a method and a system of controlling a plant for dehumidifying and/or drying plastic materials in granular and/or micro-granular and/or powder and/or flake or similar form, intended for feeding user machines associated with such a plant, in particular machines for treating and transforming plastic materials, such as machines for extruding, and subsequently injection moulding and/or blow moulding and/or compression moulding the plastic materials.
It is known, in the plastic materials moulding field, to associate a plant for dehumidifying and/or drying with at least one user machine, for example an injection moulding machine, for treating i.e. dehumidifying and/or drying, by a process fluid, the plastic material, before the latter is fed to the, at least one, user machine.
The latter generally comprises:
a melting device, also known as an extruder, for melting the plastic material, which includes, for example, an electrically heated extrusion cylinder and an extruding/plastifying screw that is rotatably movable in the cylinder;
a moulding device, in flow communication with the extruding cylinder, for moulding, for example by injection moulding and/or blow moulding and/or compression moulding, the plastic material in melted state; and
a control and management unit.
This user machine can, optionally, also be provided with a pressure transducer configured to detect an injection pressure value of the plastic material in melted state exiting the extruding cylinder of the melting device, i.e. entering the moulding device.
Plants for dehumidifying and/or drying plastic materials, in particular in granular form, associated with the, at least one, user machine disclosed above, generally comprise:
a generator of process fluid, typically air; and
at least one dehumidifying/drying hopper, intended for containing the plastic material in granular form and feeding a respective user machine.
The generator of process fluid, also known as a “dryer”, dehumidifies, heats, and delivers, a defined flow rate of process fluid in the at least one dehumidifying/drying hopper so as to treat the plastic material before the plastic material is fed to the, at least one, user machine.
In use, it may happen that the pressure detected by the pressure measurer and registered by the control and management unit during a moulding cycle falls below or rises above a preset nominal limit value, which could compromise the mechanical and/or electrical and/or chemical-physical and/or optical properties of the finished product.
For example, in the case of injection moulding machines for moulding preforms made of polyethylene terephthalate (PET), a detected pressure that is greatly below the nominal limit value may indicate a degraded melted plastic material, whilst a detected pressure that is greater than the nominal limit value may indicate high viscosity of the melted plastic material or a difficulty of the melted plastic material in reaching the moulding device, for example because of conduits that are too narrow or are blocked.
When the pressure detected by the pressure transducer falls below, or rises above, the preset nominal limit value, the control and management unit that controls the user machine, if suitably configured and enabled, can stop the user machine and thus interrupt production of the finished product.
As such user machines are not generally manned by an operator, at night it may occur that nobody realizes this or much time elapses before an operator intervenes.
Once the operator becomes aware of the stop of the user machine, the operator, in order to try to restore the correct injection pressure, acts on the user machine, for example by:
acting on the user machine cycle times, for example by acting on the loading time of the plastic material into the melting device (i.e. into the extrusion cylinder containing the extruding/plastifying screw); and/or
acting on the injection time and/or injection speed or transfer speed of the melted plastic material; and/or
acting on the plastifying temperature of the melted plastic material; and/or
acting on the compacting and/or maintenance time of the melted plastic material; and/or
acting on the pressure limits of the melting device; and/or
increasing the driving torque or the number of revolutions, of the extruding screw; and/or
acting on a historical basis of preceding episodes or according to the know-how of the company that manufactures/uses the user machine.
A drawback of this intervention method is that it entails lengthy downtime of the user machine, with consequent increase in costs due to production loss, and, before the user machine is stopped, manufacture of numerous reject products.
A further drawback of this method of intervention is that it is anyway ineffective if the plastic material provided by the plant for dehumidifying and/or drying is of low quality, for example because of the presence of humidity in the mass of the plastic material, in particular in the mass of granules. In this case, the operator can choose between two alternatives: making the user machine process all the low quality material, which is still present in the hopper, with certain rejection of the final products obtained from this material; or interrupting production, i.e. maintaining the user machine stationary and waiting for the plant for dehumidifying and/or drying to change the process conditions (so as to improve the quality of the plastic material supplied to the user machine), with a significant increase in the costs linked to the interruption of production.